Recently, there has been evidence that chocolate may lower blood pressure. However, that's not to say that it can't cause a rise in blood pressure as well. How chocolate affects you depends on your sensitivity to the ingredients in it, as well as the amount you eat.
Chocolate, when eaten in small amounts, can reduce blood pressure. A 2006 study conducted by researchers at the University Hospital of Cologne in Germany found that individuals who ate a small amount of dark chocolate a day for 18 weeks experienced a statistically significant decline in blood pressure. The researchers gave the subjects very little chocolate -- only one-fourth of an ounce, which is approximately one bite of a chocolate bar.
Chocolate contains caffeine, a stimulant that can temporarily raise blood pressure. If you already have high blood pressure, the caffeine in chocolate is likely to raise your blood pressure to a greater degree than if you have normal blood pressure. A 3.5-oz. chocolate bar has 86 mg of caffeine, according to the U.S. Department of Agriculture. This is more than is in a 6-oz. cup of coffee. If you avoid coffee to keep your blood pressure from spiking, you should avoid chocolate as well, or at the very least limit yourself to small quantities.
The sugar in the chocolate you eat may be the culprit behind any rise in blood pressure that you experience. A 2008 study published in the Archives of Internal Medicine points to high blood sugar levels as a contributing factor in the development of high blood pressure. A 3.5-oz. "jumbo" chocolate bar has over 55 g of sugar -- easily enough to cause a blood-sugar spike. Eat chocolate with added sugar on a regular basis and you could experience blood pressure difficulties that are related to high levels of blood sugar.
If you experience a rise in blood pressure after eating chocolate, the safest thing to do is to stop eating chocolate and see your physician. You may be sensitive to the caffeine, sugar or another ingredient in the chocolate. If the rise in your blood pressure is triggered by sugar or caffeine, make sure to avoid foods containing excessive amounts of these substances. Sweetened coffee drinks contain both sugar and caffeine, and may be especially problematic.